<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>2.2CSS3画三角形和对话框</title>
    <style>
        .sanjiao{
            /*border:50px solid #ccc;*/
            border-top: 50px solid transparent;
            border-left: 0px solid #f00;
            border-bottom: 50px solid transparent;
            border-right: 50px solid #00f;

            width: 0px;
            height: 0px;
            margin: 50px auto;
        }
    </style>
</head>
<body>
  <div class="sanjiao">

  </div>
</body>
</html>